Patch "afs: fix the usage of read_seqbegin_or_lock() in afs_lookup_volume_rcu()" has been added to the 6.1-stable tree

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This is a note to let you know that I've just added the patch titled

    afs: fix the usage of read_seqbegin_or_lock() in afs_lookup_volume_rcu()

to the 6.1-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     afs-fix-the-usage-of-read_seqbegin_or_lock-in-afs_lo.patch
and it can be found in the queue-6.1 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit f49e76fda4ab61771c3f6916bb534f34f0fefb60
Author: Oleg Nesterov <oleg@xxxxxxxxxx>
Date:   Thu Nov 30 12:56:06 2023 +0100

    afs: fix the usage of read_seqbegin_or_lock() in afs_lookup_volume_rcu()
    
    [ Upstream commit 4121b4337146b64560d1e46ebec77196d9287802 ]
    
    David Howells says:
    
     (2) afs_lookup_volume_rcu().
    
         There can be a lot of volumes known by a system.  A thousand would
         require a 10-step walk and this is drivable by remote operation, so I
         think this should probably take a lock on the second pass too.
    
    Make the "seq" counter odd on the 2nd pass, otherwise read_seqbegin_or_lock()
    never takes the lock.
    
    Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
    Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
    cc: Marc Dionne <marc.dionne@xxxxxxxxxxxx>
    cc: linux-afs@xxxxxxxxxxxxxxxxxxx
    Link: https://lore.kernel.org/r/20231130115606.GA21571@xxxxxxxxxx/
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/fs/afs/callback.c b/fs/afs/callback.c
index a484fa642808..90f9b2a46ff4 100644
--- a/fs/afs/callback.c
+++ b/fs/afs/callback.c
@@ -110,13 +110,14 @@ static struct afs_volume *afs_lookup_volume_rcu(struct afs_cell *cell,
 {
 	struct afs_volume *volume = NULL;
 	struct rb_node *p;
-	int seq = 0;
+	int seq = 1;
 
 	do {
 		/* Unfortunately, rbtree walking doesn't give reliable results
 		 * under just the RCU read lock, so we have to check for
 		 * changes.
 		 */
+		seq++; /* 2 on the 1st/lockless path, otherwise odd */
 		read_seqbegin_or_lock(&cell->volume_lock, &seq);
 
 		p = rcu_dereference_raw(cell->volumes.rb_node);




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